1. Field of the Invention
The invention generally relates to fluid treatment tanks and, more particularly, to a tank that has a fluid-permeable distributor plate and that can be easily manufactured and assembled with high dimensional precision. The invention additionally relates to such a distributor plate and to a method of making a fluid treatment tank including such a distributor plate.
2. Discussion of the Related Art
Fluid-permeable plates, generally known as “distributor plates,” are widely used in a variety of fluid treatment tanks. These tanks typically are known as “pressure vessels” because the fluid may be pressurized, albeit usually at a low pressure. One such tank is a pressure vessel known as a “resin tank” of a water treatment system. The typical resin tank is a hollow cylindrical structure the interior of which defines a resin bed configured to store “resin” and water or another liquid therein. The resin may be provided in the form of a plurality of plastic, e.g., polystyrene, beads. The resin bed is separated from the bottom of the tank by a slotted, perforated, or otherwise fluid permeable distributor plate that permits bidirectional fluid flow therethrough but which prevents resin from falling through the distributor plate. A riser tube may be centrally positioned within the tank. The typical riser tube extends from the distributor plate to an upper opening in the tank through which treated liquid exits the resin tank. The tank may include a blow-molded plastic tank liner reinforced by an outer layer of fiberglass wrap.
The typical distributor plate is a unitary thermoplastic structure that is thermally welded or otherwise thermally bonded to the inner wall of the tank liner. Because materials having highly-dissimilar melting points cannot be reliably thermally bonded to one another, and for cost and ease of manufacturing considerations, the typical distributor plate is formed from the same general type of material as the pressure vessel liner to which it is bonded, most typically a high density polyethylene (HDPE).
HDPE is easy to injection-mold into virtually any desired shape, but experiences relatively low dimensional stability because it has a relatively high “shrink rate.” The shrink rate or “shrinkage ratio” is a measurement of shrinkage occurring when a molten polymer cooling in a mold contracts as its temperature drops. Shrink rate typically is described either in terms of linear distance per linear distance or percentage. Rated shrink rates, as measured in accordance with ASTM standard D955, vary significantly from material-to-material and within particular materials. HDPE, for example, has a shrink rate of 0.015 to 0.040 in/in or 1.5 to 4.0%. Unfilled Noryl® (Noryl® lacking glass or otherwise being unreinforced) on the other hand, has a shrink rate of 0.005 to 0.007 in/in or 0.5 to 0.7%, and 30% glass-filled Noryl® (Noryl® which is reinforced with glass) has a shrink rate of 0.001 to 0.003 in/in or 0.1 to 0.3%. HDPE also has a much lower viscosity than unfilled or glass-filed Noryl® and requires extremely tight tools to prevent the plastic from flowing into small gaps in the tools during the injection-molding process.
The low dimensional stability exhibited by HDPE can hinder the injection molding of relatively fine distributor plate features such as slots. Such slots typically are formed during the injection molding process by thin fins extending vertically from one of the halves of a mold. The nominal slot width may be on the order of 0.25 mm to 0.33 mm. The relatively high shrink rate of HDPE and resultant low dimensional stability can lead to significant variations of slot width between slots and even within slots in a molded distributor plate. In the most extreme example, because of the low viscosity of HDPE, variations from the endpoints of this nominal dimension of up to 0.5 mm may occur. “Flashing” may occur in these instances, resulting in a thin layer of material or “flash” completely closing some of slots, preventing fluid flow through them during subsequent operation of the system. Slots may also warp or otherwise distort. Hence, it is usually difficult or impossible to maintain close tolerances of fine features molded from a material having a high shrink rate.
High shrink rate variation also hinders precision molding fine features. That is, if a material has a hypothetical shrink rate of 4%, but that shrink rate does not vary by more than +/−0.5% from lot-to-lot or within a particular mold, die dimensions and other mold properties possibly could be designed to compensate for the predicted shrinkage while still retaining acceptable dimensional stability with relatively close tolerances of molded product features. However, such compensation is difficult or impossible for materials having a low “dimensional predictability” or dimensional stability on a highly repeatable basis due to its high “heat shrink rate variability.” The “heat shrink rate variability” of a material is defined herein as the difference between highest shrink rate and the lowest shrink rate of that material as measured in accordance with an industry accepted standard such as ASTM D955. For example, HDPE has such a high shrink rate variability (on the order of +/−2.5%) that it is difficult, if not impossible, to adequately predict for shrinkage when designing the mold and the molding process properties to prevent undesired variations in slot width, shape, and/or orientation. HDPE thus has a low dimensional predictability.
The need therefore has arisen to provide a fluid treatment tank having a distributor plate they can be easily mounted to the wall of the tank but that can be injection-molded with a high level of dimensional predictability so as to assure that the widths of the apertures in a distributor plate or other fine molded features remain within tolerances.